he said recently Doflamingo was not supposed to be a final boss but help Kaido in wano, so dressrosa was not an arc he originally was going to do I guess.
if doflamingo was not going to have his own arc Law could be connected to him and still not be a big character. Like bellamy.
i'm sure all of this happened after Oda already changed his plans for law. Dressrosa probably happened because law was too popular and to give him more focus
i think it is like:
1-law is created with a connection to doflamingo
2-doflamingo is going to be just one of Kaido's commanders
3-law becomes very popular and Oda decides to give him more focus after the timeskip
4-doflamingo gains his own arc because Oda wants to focus on Law
It could be all like you said, who really knows what Oda planned and what he later added. He says he has the strawhats from the begining. But Oda never intended to have the 7 shichibukai, and we got Robin out of one. Was she planned from the begining? Im not sure. Shes not in the early concept while Jinbe is. Then there is Klabautermann from the early concept who is nowhere to be seen now, even tho we actually saw him on Skypiea as ghost of the ship and never again later. And look how Jinbe was deleted from the early concept that was published, and only after he joined we got a picture with him....
About Law, Oda might have increased his role after his popularity grew, but to such a degree? He gets too much exposure imo, for a throwaway character. A whole arc about fulfilling the wish of his parental figure, getting such a large scale flashback. If what you say is true, Oda even expanded Doflamingos character and role becauuse of Law.
There is 2 characters that have had Strawhat qualities and vibe since the timeskip imo, Kinemon and Law. Both appeared 1st arc of the new world and are major part of the story for the past 350+ chapters, both have big flashbacks with tragic ends, you can ballpark a dream for both of them, related to the ones they lost (dawn, meaning of D), both are trying to defeat a monster tyrant, and accomplish what their dead master/parental figure failed at, and both put their full trust in Luffy doing it in their stead after they fought and failed. Kinemon even says to Kaido flat out Luffy will be at the top of the seas. Thats badass af, and incredible belief. And then you have both bound to something (crew/wife), which would naturally exclude them from Strawhat contention. So much building into them... And then Oda introduces Yamato, which is just too perfect a fit as the 10th to not join, but she just appeared like chapters ago. So who really knows what goes on in that mans head. Its fun to speculate i think, thats all